Manned submersibles were often used in deep-sea research before the development of ROVs. They still have a major role to play in deep-sea research, as nothing can replace the experiences gained by actually visiting a deep-water environment.
There are only a few operational deep-sea research submersibles. The first notable submersible was the Bathysphere, an American submersible developed in the 1930s. Technology has advanced a long way since then, with the development of the highly successful Alvin submersible which routinely conducts 150 dives a year and the French submersible Nautile (Ifremer) which can operate in 97% of the world's oceans.